1. Field of the Invention
This invention relates to a magnetooptical storage element in which recording, reproduction, erasure, etc., of information are carried out by irradiating laser beams, etc., thereto.
2. Description of the Prior Art
In recent years, magnetooptical storage elements have been greatly developed as optical memory elements attaining recording, reproduction and erasure of information. Particularly, a magnetooptical storage element in which a thin film made of an amorphous alloy containing rare-earth transition metals is used as a storage medium is advantageous in that a recorded bit is not affected by the grain boundary and the film of the magnetooptical storage element can be easily manufactured to have a large surface area. However, the above-mentioned magnetooptical storage element, in which the thin film made of an amorphous alloy containing rare-earth transition metals is used as the storage medium, cannot generally achieve photomagnetic effects (i.e., Kerr effect and Faraday effect) to a full extent, resulting in an insufficient signal-to-noise ratio (i.e., S/N) of reproduced signals.
In order to eliminate such problem, an element structure referrred to as "a reflecting-film structure" has been applied to the magnetooptical storage element as described in, for example, Japanese Patent Laid-Open Publication No. 12428/1982. FIG. 2 shows a conventional magnetooptical storage element having the reflecting-film structure which comprises a transparent substrate 101 made of glass, polycarbonate, an epoxy resin, etc., a transparent dielectric film 102 having a refractive index higher than that of the transparent substrate 101, a thin film 103 made of an amorphous alloy containing rare-earth transition metals, a transparent dielectric film 104, and a reflecting metal film 105. In this conventional magnetooptical storage element having the above-mentioned structure, the thickness of the thin film 103 is so small that when laser beams are incident upon the thin film 103, a portion of the laser beam passes through the thin film 103. Therefore, both the Kerr effect, which is achieved by reflection of the laser beams on the surface of the thin film 103, and the Faraday effect, which is achieved by transmission of the laser beams through the thin film 103 upon reflection of the laser beams on the reflecting metal film 105 after the laser beams have passed through the thin film 103, are exercised on the reproduced light, so that a Kerr rotational angle of the reproduced light superficially increases as large as several times that of a magnetooptical storage element subjected to only the Kerr effect. For example, when the magnetooptical storage element shown in FIG. 2 was composed of the transparent substrate 101 made of glass, the transparent dielectric film 102 made of AlN, the thin film 103 made of GdTbFe, the transparent dielectric film 104 made of AlN, and the reflecting metal film 105 made of Al, the Kerr rotational angle increased to 1.6.degree. superficially.
However, when the reflecting metal film 105 is made of Al as mentioned above, the coefficient of thermal conductivity of Al is so high that when laser beams are irradiated onto the thin film 103 from the transparent substrate 101 so as to attain the recording of information, heat absorption arises in the said reflecting film 105 to a great extent, making necessary increased laser power for the recording of information (i.e., resulting in a magnetooptical storage element having a low recording sensitivity).